1. Field of the Invention
This invention relates to air breathing engines and more particularly to an active combustion control device for an air breathing engine combustor. In more particularity, the present invention relates to a device that applies active combustion control technology to advanced propulsion devices and closed-loop fuel injection at sub-harmonic frequencies of the instability frequency of the combustor. Also, the present invention relates to active combustion control with a combination of open loop fuel injection and closed loop fuel injection.
2. Description of the Related Art
Many propulsion systems, such as those used in various tactical missile systems, involve an enclosed combustor. There are two basic methods for controlling combustion dynamics in a combustion system: passive control and active control. As the name suggests, passive control refers to a system that incorporates certain design features and characteristics to reduce dynamic pressure oscillations. Active control, on the other hand, incorporates a sensor to detect, e.g., pressure fluctuations and to provide a feedback signal which, when suitably processed by a controller, provides an input signal to a control device. The control device in turn operates to reduce the dynamic pressure oscillations.
The combustion characteristics of an enclosed combustor, including flammability limits, instability, and efficiency are closely related to the interaction between shear flow dynamics of the fuel and air flow at the inlet and acoustic modes of the combustor. Strong interaction, between the acoustic modes of the combustor and the airflow dynamics may lead to highly unstable combustion. Specifically, unstable combustion may occur when the acoustic modes of the combustor match the instability modes of the airflow. For such conditions, the shedding of the airflow vortices upstream of the combustor tends to excite acoustic resonances in the combustion chamber, which subsequently cause the shedding of more coherent energetic vortices at the resonant frequency. The continued presence of such vortices provides a substantial contribution to the instability of the combustion process. For a more thorough discussion, please refer to U.S. Pat. No. 5,361,710 issued to Gutmark et al. on Nov. 8, 1994, which is incorporated herein by reference.
In a jet of fluid that exits from a conduit to a surrounding medium of another fluid, sudden increase of the mass-flow leads to formation of well-defined vortices that dominate the boundary between the jet fluid and the surrounding fluid. Because these vortices help transport chunks of fluid over a large distance, the rate of turbulent mixing between the two fluids is closely linked to the dynamics of these vortices. One way to manipulate the dynamics of vortices is to modulate periodically the instantaneous mass-flux of the jet.
In combustion devices, actuators can be used to enhance combustion performance such as efficiency improvement, pollutant reduction, flammability extension, and instability suppression. Combustion apparatuses, which use actuators, have been disclosed in U.S. Pat. No. 5,428,951 issued to Wilson et al. on Jul. 4, 1995, which is incorporated herein by reference. The ""951 Patent discloses several active control devices, including loudspeakers to modify the pressure field of the system or to obtain gaseous fuel flow modulations, pulsed gas jets aligned across a rearward facing step, adjustable inlets for time-variant change of the inlet area of a combustor, and solenoid-type fuel injectors for controlled unsteady addition of secondary fuel into the main combustion zone.
The periodic shedding of vortices produced in highly sheared gas flows has been recognized as a source of substantial acoustic energy for many years. For example, experimental studies have demonstrated that vortex shedding from gas flow restrictors disposed in large, segmented, solid propellant rocket motors couples with the combustion chamber acoustics to generate substantial acoustic pressures. The maximum acoustic energies are produced when the vortex shedding frequency matches one of the acoustic resonances of the combustor. It has been demonstrated that locating the restrictors near a velocity antinode generated the maximum acoustic pressures in a solid propellant rocket motor, with a highly sheared flow occurring at the grain transition boundary in boost/sustain type tactical solid propellant rocket motors.
An apparatus and method for controlling pressure oscillations caused by vortex shedding is disclosed is in U.S. Pat. No. 4,760,695 issued to Brown, et al. on Aug. 2, 1988. The ""695 patent discloses an apparatus and method for controlling pressure oscillations caused by vortex shedding. Vortex shedding can lead to excessive thrust oscillations and motor vibrations, having a detrimental effect on performance. This is achieved by restricting the grain transition boundary or combustor inlet at the sudden expansion geometry, such that the gas flow separates upstream and produces a vena contracta downstream of the restriction, which combine to preclude the formation of acoustic pressure instabilities in the flowing gas stream. Such an inlet restriction also inhibits the feedback of acoustic pressure to the point of upstream gas flow separation, thereby preventing the formation of organized oscillations. The ""695 patent does not present a method or apparatus, which attempts to control pressure oscillations in a combustor by using sub-hannonic frequencies of the instability frequency of the combustor.
While there has been a renewed interest on active combustion control (ACC) stemming from increasingly restrictive requirements on gas turbine pollution, many of the earlier studies on ACC were motivated by the desire to improve combustion performance in rockets, ramjets, and afterburners. Past studies on active combustion control (ACC) have shown that it is possible to enhance combustion performance through fast-response closed-loop feedback control as described in McManus, K. R., Poinsot, T., and Candel, S. xe2x80x9cA Review of Active Control of Combustion Instabilities,xe2x80x9d Prog. In Energy and Comb. Sci., Vol. 19, 1993, pp. 1-29. The scope of earlier investigations, however, often remained relatively basic in nature making it difficult to transition such research results into a practical system. For a more detailed explanation, please refer to Yu, K. et al. xe2x80x9cAn Experimental Study on Actively Controlled Dump Combustorsxe2x80x9d, NATO Active Control Symposium (Braunschweig, Germany, May 8-12, 2000), which is incorporated herein by reference.
Some of the previous studies in this area include instability suppression, efficiency improvement, flammability limit extension, and pollutant reduction. These studies have opened up the opportunity to study more practical issues related to potential implementation of ACC in real systems. Current research has studied the possibility of applying the active combustion control technology to advanced propulsion devices.
A preferred embodiment of the present invention involves a closed-loop liquid-fueled active control technique, which is applied in a dump combustor to enhance its combustion performance. The method and apparatus of the current invention incorporate the requirement of critical fuel flux, effects of fuel droplet size on control, and novel controller concepts. The critical fuel flux is dependent on the fuel droplet size and initial magnitude of the instabilities. When the fuel droplet size, D0, is reduced in the controlled injection, the control efficiency for heat flux actuation increases significantly. Upon analysis, an exponential dependency on the droplet size is determined. For a moderate droplet Reynold number, the amplitude of controlled heat release for a given fuel amount was inversely proportional to the droplet size by a factor of D0xe2x88x921.4.
A preferred embodiment of the present invention involves injecting fuel pulses at sub-harmonic frequencies of the instability, thus addressing the limited actuator frequency response. This embodiment of the method and apparatus applies ACC in an air breathing engine combustor. The method of active control modulation of a flame in a combustor by injecting pulsed fuel involves feeding fluid into the combustor through the combustor inlet. For the purposes of this invention, fluid may be air, liquid or gaseous fuel. Instabilities in the combustor produce natural oscillations in the fluid flow. These natural oscillations are manifestations the instability frequency of the combustor. Next, a sensor signal is generated which truly reproduces the pressure oscillations in the combustor at a given time. The pressure oscillations detected by the sensor truly reproduce the pressure oscillations that occur in the combustor at the time of sensing. The pressure oscillations detected by the sensor may merely be the natural oscillations produced by instabilities in the combustor or the pressure oscillations detected by the sensor may be the modulated oscillations. A closed loop control device operatively coupled to the actuator and responsive to the sensor determines the harmonic frequency of the instability frequency. One or more actuators inject the pulsed fuel into the shear layer of the combustor at sub-harmonic frequencies of said instability frequency. The natural oscillations are modulated by the pulsed fuel, which creates tailored conditions in the combustor.
Another preferred embodiment of the present invention utilizes both open-loop and closed-loop control schemes to obtain enhanced performance including extension of the stable combustion zone. This embodiment of the method and apparatus applies ACC in an air breathing engine combustor. Such combustors could be used in advanced ramjets, gas turbines, or afterburners. The method of this preferred embodiment involves active control modulation of a flame in a combustor having an instability frequency by injecting pulsed fuel through two or more actuators. The system includes at least one actuator coupled to closed loop control and at least one actuator coupled to the open loop control. The first step involves feeding fluid into the combustor through the inlet. Again, for the purposes of this invention, fluid may be air, liquid or gaseous fuel. Instabilities in the combustor produce natural oscillations in the fluid flow. These natural oscillations produce the instability frequency of the combustor. Next, a sensor signal is generated which truly reproduces the pressure oscillations. The pressure oscillations detected by the sensor truly reproduce the pressure oscillations that occur in the combustor at the time of sensing. The pressure oscillations detected by the sensor may merely be the natural oscillations produced by the combustor instabilities or the pressure oscillations detected by the sensor may be the modulated natural oscillations. A closed loop control device is to operatively coupled to the closed loop actuators and is responsive to a sensor. The closed loop control device controls the amplitude of pressure oscillations in the combustor. An open loop control device is operatively coupled to the open loop actuators and the open loop control device drives these actuators at a driving frequency. The driving frequency is either harmonic or sub-harmonic of the instability frequency. The respective actuators periodically inject the pulsed fuel into the shear layer of the combustor.
An objective of a preferred embodiment of the present invention provides a sub-harmonic closed loop active combustion control device, which enables higher performance in dump combustors and as a result, enables the fuel to be burned more efficiently and provides increased thrust.
A further objective of a preferred embodiment of the present invention provides a sub-harmonic closed loop active combustion control technique which not only suppresses combustion instabilities but it could also be used to extend the lean flammability limit.
A further objective of a preferred embodiment of the present invention provides a sub harmonic closed loop active combustion control in which the instability amplitude increases with the combustor output scale.
A further objective of a preferred embodiment of the present invention provides an apparatus and method of sub-harmonic closed loop active combustion control, which is based on using a closed-loop fuel injection at sub-harmonic frequencies and provides a performance similar to that using the injection at the instability frequency.
A further objective of a preferred embodiment of the present invention provides a sub-harmonic closed loop active combustion control, which could be used to relax the requirements on actuators.
A further objective of a preferred embodiment of the present invention provides an active combustion control apparatus and method, which uses a combination approach bringing an open- and closed-loop controls simultaneously and acts as a pacemaker when there is a significant drift in the natural instability frequency.
A further objective of a preferred embodiment of the present invention provides an active combustion control apparatus and method that can suppress unwanted oscillations and extend the flammability limit.